1. Field of the Invention
This invention relates to a method of stabilizing printed circuit board substrate laminates through application of a number of laminate conditioning and baking cycles.
2. Description of the Prior Art
In the present day technology to which this invention relates, the fabrication of printed circuit boards utilizes critical processes. The signal cores of such printed circuit boards are frequently made from a laminate comprising epoxy-glass or like material in the order of from 3 to 20 mils thick exclusive of a copper layer on both sides of the core laminate. Generally, the core is fabricated by etching the copper from both sides of the laminate except for a small border. A plurality of interconnection through-holes, commonly called "vias," are drilled in the core laminate. This is followed by pattern plating the proper circuits on each side of the core laminate. If the "vias" do not align properly with the electrical interconnecting pads at the end of the signal lines forming the circuit patterns, the electrical interconnections between the signal circuits cannot be established.
U.S. Pat. No. 3,210,214 relates to a method of forming precise electrical conductive patterns on insulative substrates. The process includes an annealing of the substrate to eliminate the formation of strains within the substrate. U.S. Pat. Nos. 3,708,876 and 3,755,890 both relate to the vacuum-heat treatment of printed circuit boards to drive off vaporizable matter. U.S. Pat. No. 3,508,330 relates to the method of making integral conductor paths and through-holes in multilayer boards.
Empirically, it was learned that even though the holes have been drilled in the laminate cores at their proper locations, by the time that the signal line patterns were defined and created on the core laminates, the "vias" did not register with the electrical interconnecting pads. An investigation into the causes for this dimensional shift revealed an irreversible dimensional change in the core as a result of the etching of the copper from the core laminate. There is also a reversible expansion and contraction of the core due to humidity. The criticality of the dimensional changes is proportional to the size of the core laminate.